A steam turbine plant is mainly provided with a high-pressure steam turbine in which main steam performs work; an intermediate-pressure steam turbine in which reheated steam performs work; and a low-pressure steam turbine in which steam discharged from the intermediate-pressure steam turbine performs work. Among these steam turbines, the low-pressure steam turbine is coupled to a condenser, and the steam discharged from the low-pressure steam turbine is condensed in the condenser so as to generate condensate.
An outer casing of a low-pressure steam turbine is a pressure vessel. From a viewpoint of ease in assembly and disassembly, the outer casing is divided into two parts, an outer casing upper half and an outer casing lower half, by a horizontal plane including a shaft center line of a turbine rotor. A flange of the outer casing upper half and a flange of the outer casing lower half are fastened to each other with a bolt and the like. A foot plate is provided to a side surface close to the flange of the outer casing lower half. This foot plate is fixed to a foundation, and the outer casing is supported on the foundation by the foot plate.
An outer portion of the outer casing in the low-pressure steam turbine is exposed to the atmosphere, while an inner portion thereof is caused to be in a vacuum state by the condenser. Accordingly, the outer casing receives a load due to a difference between pressure applied to an outer surface and pressure applied to an inner surface. Typically, this load is called a vacuum load. When receiving a vacuum load, the outer casing may deform to recess inward. Therefore, an inner casing supported by the outer casing lower half may be displaced as being affected by deformation of the outer casing due to the vacuum load.
On the other hand, the turbine rotor is rotatably supported by a rotor bearing. This rotor bearing is supported by a bearing base. A cone is provided to a central part of an end plate of the outer casing. This cone protrudes from the end plate toward the inside of the outer casing. The bearing base is typically supported by this cone. Therefore, when the rotor bearing receives a load from the turbine rotor, the load is transferred to the outer casing through the bearing base, which may deform the outer casing. Accordingly, the rotor bearing may be displaced. Furthermore, since the bearing base is supported by the outer casing, there is a possibility that the rotor bearing may be displaced by deformation of the outer casing due to the vacuum load.
In this manner, displacement of the rotor bearing may lead to displacement of the turbine rotor as a rotary unit. As described above, the inner casing as a stationary unit may be displaced due to an influence deformation of the outer casing due to the vacuum load or the load from the turbine rotor. Therefore, in consideration of the aforementioned positional displacement, it is difficult to reduce a gap between the rotary unit and the stationary unit in order to prevent contact between the rotary unit and the stationary unit. Such a case increases detriment attributable to steam leaking from between the rotary unit and the stationary unit, which may degrade performance of the turbine.